She's not strong at all points of the game. Her early game is honestly atrocious which sucks because her mid game is even worse. She gets bullied really hard early game due to having really awful range and no meaningful ways of trading. Either you go in and murder them or you go in and you die. And if you dont snowball your lane, your lack of any ability damage for poking with means that your midgame in skirmishes just sucks ass because all you have are AAs. AAs that dont do damage yet because you dont have items.
By late game is the only time she's good and that's when she should be good. If she's doing damage late, there shouldn't be a reason for you to complain. Her power trough is deep and rocky enough that if, in this meta, you haven't finished the game out before she hits your stride, that's on you. Or you opened up a soup kitchen in laning phase and just force fed her the entirety of it. Either way, both are your fault.
People need to figure out that just because a champion is safe doesn't mean they are good. If it did, then ezreal is a much worse case of a champion who is "good" at all stages of the game. His mobility is a blink and its CD is reduced just for him landing his abilities. He literally fits with any meta and any support and any playstyle; wanna just poke? Can do. Wanna just farm? Also viable. Wanna go ham and all in? Got that covered too. And now, with his rework, he can also siege. Dude doesnt fall off, has one of the highest damage numbers per game for an ADC, and has a very safe early game, extremely good mid game, and a decent late game.
But yes, Tristana with long CDs, no means of poking, expensive build path, extremely weak early and mid game, and limited means of interacting with her laner in lane is the worst thing on the planet.